Purely acoustic measuring methods are used primarily in the prior art, with the aid of which a bonded substrate stack can be investigated with regards to the structure and/or faults thereof. For the most part, the entire substrate stack is placed into a liquid, particularly water, to couple in a sound signal.
In addition, electromagnetic measuring methods and plants also exist, with the aid of which one can measure a substrate stack. In this case, electromagnetic beams are introduced into the substrate stack and the reflection thereof is measured. One such method is disclosed in WO2012062343A1.
Wetting the substrate stack with liquid constitutes the greatest problem. Wetting of this type is undesirable for several reasons.
First, the substrate stack must also be dried after removal from the plant and second, liquid can penetrate into the boundary surface between the substrates.
The electromagnetic measuring methods have the disadvantage that the information is limited by the limited penetration depth of the electromagnetic beams. The beams penetrate only to a relatively short depth of the substrate stack, in particular owing to metallic components with high absorption. Furthermore, the reflected measured signal is severely weakened after exiting the substrate stack. Furthermore, for the most part scanning methods are used, that is to say methods in which the substrate stack completes a relative movement with respect to a sharply focused beam which is scanning the substrate stack. This relative movement is extremely time-intensive and therefore costly.